1. Field of the Invention
The present invention relates to switched-capacitor filter circuits and in particular to such circuits having at least one simulated inductor which contains switches, capacitors and amplifiers controlled according to predetermined clock pulses and in which an operational amplifier is provided between whose output and inverting input a capacitor is connected and whose non-inverting input is connected to a fixed reference potential.
2. Description of the Prior Art
Switched-capacitor (SC) filters are known from the article "Switched-Capacitor Filter Design Using The Bilinear z-Transform" in the periodical "IEEE Transactions on Circuits and Systems," Vol. Cas-25, No. 12, December 1978, pages 1039 through 1044 as well as from the article "Switched-Capacitor Circuits Bilinearly Equivalent To Floating Inductor Or F.D.N.R." in the periodical Electronics Letters, Feb. 1, 1979, Vol. 15, No. 3, pages 87 and 88. Such filters do not process time-continuous analog signals in the true sense, but rather process time-discrete signals which exist in the form of samples, which samples are generated according to a clock frequency F via the relationship T=1/F, where T is the clock period. Circuits for generating such samples are known to those skilled in the art and are not explained in detail herein. Such sampling circuits can be pre- or post-connected to the known circuits illustrated, so that samples derived from an analog signal are supplied to the filter circuit at its input side and the signals available at the output side are re-converted into time-continuous analog signals. The significant, technical advantage of such filters is that coils are simulated by means of active circuit elements and capacitors, which are suitable for the monolithic integration of larger filter circuits. Known operational amplifiers are predominantly employed as amplifiers and one thereby strives, on the one hand, to require the smallest possible number of circuit elements and, on the other hand, to also guarantee the stability of such circuits.
In the aforementioned known circuits, the inverting input is also connected via a capacitor to the output of an operational amplifier, so-called "counter-coupled." Some operational amplifiers employed are at times not counter-coupled or may require a high common mode rejection, because the inverting input of the operational amplifier is briefly not counter-coupled during certain switching phases or the non-inverting input is not always held at grounded potential. Further, if the realization of the capacitors ensues by metal-oxide-semiconductor (MOS) technology the unavoidable ground capacitance connected with every floating MOS capacitor can lead to significant disruptions of the filter function.